www.google.com/googlevoice/about.html
Go to Google Voice

Google Voice gives you a single phone number that rings all of your phones, saves your voicemail online, and transcribes your voicemail to text. Other cool features include the ability to listen in on messages while they're being left, block unwanted callers, and make cheap international calls.

Features:
Set up profile for which phone to ring based on caller or time of day
Voicemail transcription (works sometimes)
Call from Google voice website - It will call you then connect you with the called party.
SMS forwarding to email

• Ask unknown callers to give their name (option)
• Hear caller ID before accepting call
• Set Caller ID to displan caller's # or your Google voice number
• Do not disturbe button
• Read transcriptions of voicemails. Watch a video
• Customize which phones ring. Watch a video
• Personalize greetings for different callers. Watch a video
• Make cheap international calls. Watch a video
• Forward SMS to email. Watch a video
• Share voicemails with friends. Watch a video
• Block unwanted callers. Watch a video
• Screen callers before answering. Watch a video
• Access the mobile app on your phone. Watch a video
• Conference call with co-workers. Watch a video

My Tips:

To change settings- add a phone ... 
   1. Sign in to Google Voice.
   2. Click the gear icon at the top of the page. gear icon
   3. Click Voice settings.
   4. Select the Phones tab.
   "Add another phone" is at the bottom.
   Change Settings:
   5. Click the Edit button under the phone for which you want to set a ring schedule.
   6. Click the Show advanced settings link.
   7. In the Ring Schedule section, select your option using the radio buttons. To make
   sure a phone doesn't ring at a specific time of the day, select Use custom schedule and
   enter your time range.
   8. Click Save at the bottom of the page.
   
To set up a call to dial:
Go to Google Voice (www.google.com/voice/)
Click Call
Specify a name in your contact list or a number to call
Specify phone to call with.
   If you specify Google Talk you must have a gmail window open.
   The google talk page says it doesn't work on a Mac, but it uses google chat instead.
   

2. It will only ring a phone for 25 seconds before going to voice mail. This setting cannot be changed.
The answering service on my home line is set to 4 rings (30 seconds) before going to voice mail, so it never goes to the voice mail on my home phone, unless I reduce the number of rings.

Links:
Sign in/sign up (www.google.com/voice/)
Make Free VoIP Calls from Google Voice using a sipgate number.
Unlimited Free Calling with Google Voice - Google Docs
Christina Trapolino comments on her Google Voice experiment - Google+
Google Voice Quick Reference Cheatsheet v2.0 Speeds Through Phone Menus
Google Voice at Google Docs